The <?xml version="1.0"?> Declaration for HTML

<?xml?> Declaration Syntax

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<?xml-stylesheet ...?>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
   ...
</html>

Rules for coding the xml declaration for HTML 5

Code the <?xml?> tag on the first line of the document, before anything else. When generating code using server-side scripting languages such as PHP, it should be the first thing that is output, after the HTTP headers.
The xml declaration starts with the characters <?, which indicates that the markup is a SGML declaration rather than an HTML tag.
Following the <? delimiters, code the letters xml in lower case.
Include a version attribute to indicate the characters that can be used in element names, and an encoding attribute to specify the character encoding of the document.
Include a standalone attribute with the value "yes" to indicate that the document is a standalone document and is not to be affected by any external declarations.
The xml declaration ends with the characters ?>.
The <?xml?> tag does not have a matching end tag.

The following rules apply to the entire HTML document, based on the <?xml?> declaration:

If the version attribute specifies XML version 1.1, any characters in the range  through  must be encoded as HTML character entities, in addition to all characters in the range  through . The NUL character &x#00; is not permitted in either version of XML.